Tube-wave monitoring as a method to detect shear modulus changes around boreholes: A case study

Abstract

Monitoring the shear modulus of formations around boreholes is of interest for various applications, ranging from near-surface investigation to reservoir monitoring. Downhole logging tools and borehole seismic are common techniques applied to measure and characterize formation properties. These methods rely on transmitted and reflected waves to retrieve the rock properties. Wave modes traveling along the interface between the well and the formation, such as tube waves, are often considered as noise. However, tube waves are less attenuated than body waves and contain information about the shear modulus of the formation surrounding the well. Hence, a potential use of this interface wave is of interest. Because tube-wave properties depend on several parameters, e.g., well geometry, highly accurate measurements should be performed for use in inferring rock properties. We have studied the feasibility of tube-wave measurements as a monitoring method. Different experiments are conducted using a hydrophone array in two boreholes, with depths of 30 and 95 m. The experiments are used to investigate how accurately the tube-wave velocity can be measured and which parameters have the most impact on the measurements. Our results suggest that it is hard to estimate the absolute shear modulus of the geologic formation using tube-wave velocities only. However, it seems feasible to use them to monitor changes in the shear modulus, depending on the borehole setup and the geologic formation. Tube-wave monitoring can be used as a first step to determine the depth along the well where changes occur before more accurate measurements are performed in a second step.